338 JACQUES LE MAGNEN 



more useful differentiation of the gustatory from the olfactory apparatus 

 than do anatomical and other functional criteria, and gives an exact 

 definition of the former. 



In olfaction, as in taste, chemical differentiation depends on a supply of 

 information from the external environment. But in olfaction this analysis 

 is carried out both quantitatively and qualitatively at the molecular level, 

 quantitatively by the threshold activity of molecular units, qualitatively 

 with regard to the possibility of identifying a single molecular unit. Apart 

 from certain cases where confusion occurs, every active molecule is sepa- 

 rated from others by a specific quality— its own odour. Olfaction is the 

 sensation of chemical " individualization ", an improved definition first 

 introduced by H. Pieron. Quantitatively, and even more so qualitatively, 

 molecular analysis is the basis of the olfactory sense. At the same time this 

 analysis of chemical units and mixtures is not, of course, perfect. The 

 information supplied to the central nervous system by means of the olfac- 

 tory pathway as to the chemical structure of the environment is subjected 

 to a certain amount of distortion. Both in animals, whose discriminatory 

 spectrum has been determined, and in the human being, on the basis of 

 objective data, olfactory discrimination does not coincide with the strictly 

 chemical differentiation. The sensory qualitative unit does not correspond 

 exactly to the chemical units and mixtures. Where chemical units (definite 

 atoms and molecules) are concerned, olfactory analysis sometimes falls 

 short of the molecular level and sometimes goes beyond it. The first of 

 these conditions applies in some cases where molecules of different chemi- 

 cal structure (e.g. nitrobenzene, benzaldehyde, the musks, etc.) are con- 

 fused; as in taste, appearing to have the same quality. The second condi- 

 tion probably applies where, as shown in human olfactory discrimination, 

 the smell of a given molecule may still be analysed in a mixture of different 

 qualitative components (e.g. the smell of isoborneol perceived together with 

 both camphor and musty odours). 



The analysis of the kind of chemical mixtures which form most of the 

 biological stimuli affecting behaviour (foods, body odours, etc.) suffers the 

 same kind of distortion. Sometimes the mixture cannot be analysed 

 olfactively. This is the case, in humans, for most natural products such as 

 vegetable and animal odours. Smells like those of an essential oil, a vege- 

 table oil, or the flavour of coffee, which are very complex chemical mix- 

 tures, appear as specific odours which cannot be analysed. I do not know 

 of any experiment in animals which demonstrates that the response to a 

 mixture is given as if to a single total stimulus ; but such a finding is highly 

 probable and would be easy to confirm by adulteration of these mixtures. 

 With some mixtures, on the other hand, both in humans and animals, an 

 olfactory analysis of the components does occur. In humans this is 

 generally the case with synthetic mixtures, such as commercial perfume. 



